Conventionally, as a facsimile apparatus capable of performing communication at a plurality of communication speeds, a G3 standard facsimile apparatus defined in the CCITT recommendation T4 is known. A procedure of determining a communication speed in the conventional G3 standard facsimile apparatus will be described below.
In the G3 standard facsimile apparatus, prior to high-speed image transmission, an automatic equalizer of a reception-side modem is adjusted to be matched with line characteristics using a training signal and a training check signal. The adjustment result is discriminated, and the reception-side apparatus transmits, to a transmission-side apparatus, a signal representing that training is successful or a signal indicating that training is unsuccessful and re-training is required.
FIGS. 20(A,B) and 21 show communication control procedures of a general G3 standard facsimile apparatus.
In each of FIGS. 20(A,B) and 21, signals on the left-hand side of the central line are transmitted from a transmission-side apparatus (calling station) T, and signals on the right-hand side of the central line are transmitted from a reception-side apparatus (called station) R.
In FIGS. 20(A,B) and 21, an NSF (non-standard device) signal, a CSI (called station identification) signal, and a DIS (digital identification) signal are initial identification signals, and are used such that the reception-side apparatus informs its own facsimile functions to the transmission-side apparatus of a station on the other end of the line.
An NSS (non-standard device setting) signal, a TSI (transmission station identification) signal, and a DCS (digital command) signal are reception command signals transmitted from the transmission-side apparatus to designate a transmission mode to be used. When the transmission-side apparatus transmits a training check (TCF) signal after it transmits these signals and the reception-side apparatus sends back a CFR (reception ready confirmation) signal, a transmission speed of the following image signal to be transmitted is also designated.
The training check (TCF) signal is set to the reception-side apparatus R through a modulation system complying with the G3 standards, so that the reception-side apparatus R checks a training signal, and indicates whether or not a channel can be used at the designated speed. The format of the TCF signal is a continuous "0" signal for 1.5 sec.+-.10%. The training signal transmitted immediately before the TCF signal is a sync signal for appropriately adjusting a reception-side modem. This sync signal is used for carrier detection, AGC (automatic gain control), timing synchronization, synchronization of a descrambler, and convergence of an equalizer (to be described later) if necessary.
The CFR (reception ready confirmation) signal and an FTT (train failure) signal are pre-message response signals transmitted from the reception-side apparatus.
The reception ready confirmation signal (CFR) is a digital response signal for confirming that all the pre-message procedures are completed and message transmission is ready to start.
The train failure (FTT) signal is an optional digital response signal for requesting to delete all or some of the pre-message procedures and to start re-training of a modulation system complying with the G3 standards.
A PIX signal is an image signal as a message. Immediately before transmission of this image signal, the training signal is transmitted.
An EOP signal is a procedure end signal. The EOP signal indicates an end of one page of facsimile information, and also indicates that control returns to the beginning of a phase B (pre-message procedure) of procedures.
An MCF signal is a message confirmation signal indicating that all the messages are received and an additional message can follow.
In the above description, when digital signal data is transmitted through a general public line as an analog line, the digital signal must be transmitted after it is modulated to be converted to a predetermined analog signal. A reception-side apparatus must demodulate the modulated signal, and a modem is necessary for this purpose.
A transmission speed upon data transmission tends to be increased. In some G3 facsimile systems, data is transmitted at a high data transmission speed of 9,600 bps (bits/sec). For this reason, a transmission signal modulated by a transmission-side modem and sent onto a line is distorted due to a line distortion, jitter, a timing error between transmission- and reception-side apparatuses, a carrier error, and the like, and is received by a reception-side modem.
The reception-side modem is assembled with an equalizer or the like for correcting the distortion. Before the received signal is output from the reception-side modem, it is corrected to be the original transmission signal.
A general operation of this equalizer will be described below with reference to FIG. 21 and FIGS. 22A to 22C.
In FIG. 21, reference numeral 10 denotes a transmission-side modem; 20, a reception-side modem; 21, an equalizer assembled in the reception-side modem 20; and 30, a line connecting both the modems.
A transmission signal a.sub.k output from the transmission-side modem 10 is sent onto the line 30 to have a uniform gain over the entire use range, as shown in FIG. 22A. However, the line 30 has frequency characteristics, and its transmission characteristics are as shown in FIG. 22B.
For this reason, a reception signal R.sub.k received at the reception-side modem 20 is influenced by the transmission characteristics, as shown in FIG. 22B. The reception-side modem 20 comprises the equalizer 21 having frequency characteristics shown in FIG. 22C, so that an output signal a.sub.k from the equalizer 21 has synthesized characteristics of FIGS. 22B and 22C. Both the frequency characteristics are opposite characteristics, and hence, the characteristics [those obtained by convoluting both the characteristics (a simple multiplication in a frequency region)] of the output signal a.sub.k become flat, as shown in FIG. 22A.
As a result, signal transmission free from distortion is achieved.
That is, the role of the equalizer 21 is to generate characteristics opposite to line characteristics.
With the above-mentioned method, in data communication such as facsimile communication using a public telephone line, line characteristics are equalized.
The table below shows training patterns for the equalizer which are transmitted immediately before the TCF signal and are recommended by the CCITT. V29 recommendation, and a fall-back mode will be described below with reference to the CCITT. V29 recommendation.
Note that segment 2 in the table below is a pattern for matching timing phases, and segment 3 is a pattern for adjusting the equalizer.
The table shows sync signals of the V29 recommendation, which are used for equalizing line characteristics prior to transmission/reception of facsimile image data.
Following the sync signal, a continuous "0" signal called a TCF (training check) is sent for 1.5 sec.+-.10% so as to confirm whether or not line characteristics can be satisfactorily equalized.
TABLE ______________________________________ Number of Approximate Type of Symbol Processing Line Signal Intervals Time: (ms) ______________________________________ Segment 1 No Energy 48 20 to be Transmitted Segment 2 Alternate 128 53 Segment 3 Pattern for 384 160 Adjusting Equalizer Segment 4 Scrambled 48 20 Data "1" Total of Total of 609 253 Segments Sync Signals ______________________________________
In FIGS. 20(A,B) and 21, both the transmission-reception-side apparatuses comprise modems having a function of transmitting data at transmission speeds of 2,400 bps, 4,800 bps, 7,200 bps, and 9,600 bps.
Therefore, the transmission-side apparatus first sends a sync signal having a transmission speed of 9,600 bps, and then sends the TCF.
The reception-side apparatus performs AGC control, timing extraction, equalizer adjustment, synchronization of a descrambler, and the like during reception of the sync signal.
During a TCF period, e.g., for 1 sec, it is checked if there is an error. If an error is detected, the reception-side apparatus sends back an FTT (training failure) signal to the transmission-side apparatus, and falls back the transmission speed to 7,200 bps. If no error is detected, the reception-side apparatus transmits a CFR (reception confirmation signal) to the transmission-side apparatus, and image data transmission/reception is performed at 9,600 bps without falling back a transmission speed.
FIG. 20A exemplifies that the transmission-side apparatus tries to transmit data at a transmission speed of 9,600 bps, the reception-side apparatus sends the CFR signal since it can correctly receive the TCF signal, and then image transmission is performed at 9,600 bps.
A function of judging whether the reception-side apparatus transmits the CFR or FTT signal after it receives the TCF signal is not standardized, and varies depending on individual apparatuses. Thus, there is a room for manufacturers to show features in their models.
For example, a demodulated TCF signal is checked, and when "0" data can be continuously received for 1.0 sec or more, the CFR signal is transmitted; otherwise, the FTT signal is transmitted.
FIG. 21 exemplifies a case of a poor line condition.
More specifically, although the transmission-side apparatus tries to transmit data at 9,600 bps, the reception-side apparatus cannot converge an equalizer due to the poor line condition and cannot correctly receive the TCF signal. In this case, the reception-side apparatus transmits the FTT signal. The transmission-side apparatus tries to transmit data at 7,200 bps upon reception of the FTT signal. Since the reception-side apparatus cannot correctly receive the TCF signal yet, it transmits the FTT signal again. For this reason, the transmission-side apparatus tries to transmit data at 4,800 bps. Since the reception-side apparatus cannot correctly receive the TCF signal yet, it transmits the FTT signal again.
When the transmission-side apparatus receives the FTT signal for the TCF signal at a transmission speed of 9,600 bps or 7,200 bps as described above, it immediately transits to transmission processing of the TCF signal for 7,200 bps or 4,800 bps.
However, for the TCF signal at 4,800 bps or 2,400 bps, when the FTT signal is received twice, the control transits to transmission processing of the TCF signal from 4,800 bps to 2,400 bps or from 2,400 bps to disconnection of the line for the following reason.
That is, at 4,800 bps or 2,400 bps, transmission is tried to be performed at that speed if possible.
Since the transmission-side apparatus performs such communication control, it tries to transmit data at 4,800 bps again after reception of the first FTT signal at 4,800 bps. When the reception-side apparatus cannot correctly receive the TCF signal yet, it transmits the FTT signal. At this time, since the transmission-side apparatus has received the FTT signal twice for transmission of the TCF signal at 4,800 bps, it then tries to transmit data at 2,400 bps.
For example, in FIG. 20A, since the reception-side apparatus can correctly receive the TCF signal at 2,400 bps first, it transmits the CFR signal. For this reason, the transmission-side apparatus transmits an image signal at 2,400 bps.
However, the conventional system has the following problem for a line condition check signal (TCF signal) for checking whether or not an image signal can be transmitted at the designated transmission speed. That is, the reception-side apparatus can only make two judgments, i.e., whether or not the reception-side apparatus can perform transmission at the designated transmission speed, and whether or not transmission is impossible at the designated transmission speed.
More specifically, as shown in FIG. 20B, when the line condition is poor, the transmission speed falls back from 9,600 bps to 7,200 bps, 4,800 bps, and 2,400 bps in turn. Therefore, a very long processing time is required for pre-procedures.
Therefore, when the transmission line characteristics are poor, an effective transmission speed using a V27ter modem (4,800 bps, 2,400 bps) is often higher than that using a V29 modem (9,600 bps, 7,200 bps).
In future, high-speed modems having transmission speeds of 12.0 kbps, 14.4 kbps, and 19.2 kbps will be recommended. However, if a fall-back mode like in the conventional system is used in the high-speed modems, the effective transmission efficiency would be further impaired.
For example, even if both the transmission- and reception-side apparatuses have a function of transmitting data at a transmission speed of 19.2 kbps, when a connected line condition is unexpectedly poor and transmission is performed at a transmission speed of 2,400 bps, if a transmission speed falls back one by one every time the FTT signal is received like in the prior art, the pre-procedures take about 42 sec until it ends (the reception-side apparatus transmits the CFR signal).
Recently, a so-called asymmetrical modem ("asymmetrical" means that an upstream transmission speed is not always equal to a downstream transmission speed) has been developed. As a simplest one of the asymmetrical modems, a tonal signal is known as a backward signal.
For example, when a 14.4-kbps modem is used, a time required for transmitting one page of an A4 standard original is 6 sec. Therefore, in consideration of this transmission time, it is wasteful to transmit a line condition check signal (e.g., the TCF signal) for checking whether or not a channel can be used at the designated transmission speed prior to transmission of image data. Thus, a further problem to be solved is posed.